A typical fixed-cutter, or “drag,” rotary drill bit for drilling subterranean formations includes a bit body having a face region thereon carrying cutting elements for cutting into an earth formation. The bit body may be secured to a hardened steel shank having a threaded pin connection for attaching the drill bit to a drill string that includes tubular pipe segments coupled end to end between the drill bit and other drilling equipment. Equipment such as a rotary table or top drive may be used for rotating the tubular pipe and drill bit. Alternatively, the shank may be coupled directly to the drive shaft of a down-hole motor to rotate the drill bit.
Typically, the bit body of a drill bit is formed from steel or a combination of a steel blank embedded in a matrix material that includes hard particulate material, such as tungsten carbide, infiltrated with a binder material such as a copper alloy. A steel shank may be secured to the bit body after the bit body has been formed. Structural features may be provided at selected locations on and in the bit body to facilitate the drilling process. Such structural features may include, for example, radially and longitudinally extending blades, cutting element pockets, ridges, lands, nozzle displacements, and drilling fluid courses and passages. The cutting elements generally are secured within pockets that are machined into blades located on the face region of the bit body.
Generally, the cutting elements of a fixed-cutter type drill bit each include a cutting surface comprising a hard, super-abrasive material such as mutually bound particles of polycrystalline diamond. Such “polycrystalline diamond compact” (PDC) cutters have been employed on fixed-cutter rotary drill bits in the oil and gas well drilling industries for several decades.
FIG. 1 illustrates a conventional fixed-cutter rotary drill bit 10 generally according to the description above. The rotary drill bit 10 includes a bit body 12 that is coupled to a steel shank 14. A bore (not shown) is formed longitudinally through a portion of the drill bit 10 for communicating drilling fluid to a face 20 of the drill bit 10 via nozzles 19 during drilling operations. Cutting elements 22 (typically polycrystalline diamond compact (PDC) cutting elements) generally are bonded to the face 20 of the bit body 12 by methods such as brazing, adhesive bonding, or mechanical affixation.
A drill bit 10 may be used numerous times to perform successive drilling operations during which the surfaces of the bit body 12 and cutting elements 22 may be subjected to extreme forces and stresses as the cutting elements 22 of the drill bit 10 shear away the underlying earth formation. These extreme forces and stresses cause the cutting elements 22 and the surfaces of the bit body 12 to wear. Eventually, the cutting elements 22 and the surfaces of the bit body 12 may wear to an extent at which the drill bit 10 is no longer suitable for use.
FIG. 2 is an enlarged view of a PDC cutting element 22 like those shown in FIG. 1 secured to the bit body 12. Cutting elements 22 generally are not integrally formed with the bit body 12. Typically, the cutting elements 22 are fabricated separately from the bit body 12 and secured within pockets 21 formed in the outer surface of the bit body 12. A bonding material 24 such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements 22 to the bit body 12 as previously discussed herein. Furthermore, if the cutting element 22 is a PDC cutter, the cutting element 22 may include a polycrystalline diamond compact table 28 secured to a cutting element body or substrate 23, which may be unitary or comprise two components bound together.
The bonding material 24 typically is much less resistant to wear than are other portions and surfaces of the drill bit 10 and of cutting elements 22. During use, small vugs, voids and other defects may be formed in exposed surfaces of the bonding material 24 due to wear. Solids-laden drilling fluids and formation debris generated during the drilling process may further erode, abrade and enlarge the small vugs and voids in the bonding material 24. The entire cutting element 22 may separate from the drill bit body 12 during a drilling operation if enough bonding material 24 is removed. Loss of a cutting element 22 during a drilling operation can lead to rapid wear of other cutting elements and catastrophic failure of the entire drill bit 10. Therefore, there is a need in the art for an effective method for preventing the loss of cutting elements during drilling operations.
The materials of an ideal drill bit must be extremely hard to efficiently shear away the underlying earth formations without excessive wear. Due to the extreme forces and stresses to which drill bits are subjected during drilling operations, the materials of an ideal drill bit must simultaneously exhibit high fracture toughness. In practicality, however, materials that exhibit extremely high hardness tend to be relatively brittle and do not exhibit high fracture toughness, while materials exhibiting high fracture toughness tend to be relatively soft and do not exhibit high hardness. As a result, a compromise must be made between hardness and fracture toughness when selecting materials for use in drill bits.
In an effort to simultaneously improve both the hardness and fracture toughness of earth-boring drill bits, composite materials have been applied to the surfaces of drill bits that are subjected to extreme wear. These composite materials are often referred to as “hard-facing” materials and typically include at least one phase that exhibits relatively high hardness and another phase that exhibits relatively high fracture toughness.
FIG. 3 is a representation of a photomicrograph of a polished and etched surface of a conventional hard-facing material. The hard-facing material includes tungsten carbide particles 40 substantially randomly dispersed throughout an iron-based matrix material 46. The tungsten carbide particles 40 exhibit relatively high hardness, while the matrix material 46 exhibits relatively high fracture toughness.
Tungsten carbide particles 40 used in hard-facing materials may comprise one or more of cast tungsten carbide particles, sintered tungsten carbide particles, and macrocrystalline tungsten carbide particles. The tungsten carbide system includes two stoichiometric compounds, WC and W2C, with a continuous range of compositions therebetween. Cast tungsten carbide generally includes a eutectic mixture of the WC and W2C compounds. Sintered tungsten carbide particles include relatively smaller particles of WC bonded together by a matrix material. Cobalt and cobalt alloys are often used as matrix materials in sintered tungsten carbide particles. Sintered tungsten carbide particles can be formed by mixing together a first powder that includes the relatively smaller tungsten carbide particles and a second powder that includes cobalt particles. The powder mixture is formed in a “green” state. The green powder mixture then is sintered at a temperature near the melting temperature of the cobalt particles to form a matrix of cobalt material surrounding the tungsten carbide particles to form particles of sintered tungsten carbide. Finally, macrocrystalline tungsten carbide particles generally consist of single crystals of WC.
Various techniques known in the art may be used to apply a hard-facing material such as that represented in FIG. 3 to a surface of a drill bit. A rod may be configured as a hollow, cylindrical tube formed from the matrix material of the hard-facing material that is filled with tungsten carbide particles. At least one end of the hollow, cylindrical tube may be sealed. The sealed end of the tube then may be melted or welded onto the desired surface on the drill bit. As the tube melts, the tungsten carbide particles within the hollow, cylindrical tube mix with the molten matrix material as it is deposited onto the drill bit. An alternative technique involves forming a cast rod of the hard-facing material and using either an arc or a torch to apply or weld hard-facing material disposed at an end of the rod to the desired surface on the drill bit.
Arc welding techniques also may be used to apply a hard-facing material to a surface of a drill bit. For example, a plasma transferred arc may be established between an electrode and a region on a surface of a drill bit on which it is desired to apply a hard-facing material. A powder mixture including both particles of tungsten carbide and particles of matrix material then may be directed through or proximate the plasma transferred arc onto the region of the surface of the drill bit. The heat generated by the arc melts at least the particles of matrix material to form a weld pool on the surface of the drill bit, which subsequently solidifies to form the hard-facing material layer on the surface of the drill bit.
When a hard-facing material is applied to a surface of a drill bit, relatively high temperatures are used to melt at least the matrix material. At these relatively high temperatures, atomic diffusion may occur between the tungsten carbide particles and the matrix material. In other words, after applying the hard-facing material, at least some atoms originally contained in a tungsten carbide particle (tungsten and carbon for example) may be found in the matrix material surrounding the tungsten carbide particle. In addition, at least some atoms originally contained in the matrix material (iron for example) may be found in the tungsten carbide particles. FIG. 4 is an enlarged view of a tungsten carbide particle 40 shown in FIG. 3. At least some atoms originally contained in the tungsten carbide particle 40 (tungsten and carbon for example) may be found in a region 47 of the matrix material 46 immediately surrounding the tungsten carbide particle 40. The region 47 roughly includes the region of the matrix material 46 enclosed within the phantom line 48. In addition, at least some atoms originally contained in the matrix material 46 (iron for example) may be found in a peripheral or outer region 41 of the tungsten carbide particle 40. The outer region 41 roughly includes the region of the tungsten carbide particle 40 outside the phantom line 42.
Atomic diffusion between the tungsten carbide particle 40 and the matrix material 46 may embrittle the matrix material 46 in the region 47 surrounding the tungsten carbide particle 40 and reduce the hardness of the tungsten carbide particle 40 in the outer region 41 thereof, reducing the overall effectiveness of the hard-facing material. Therefore, there is a need in the art for abrasive wear-resistant hardfacing materials that include a matrix material that allows for atomic diffusion between tungsten carbide particles and the matrix material to be minimized. There is also a need in the art for methods of applying such abrasive wear-resistant hardfacing materials, and for drill bits and drilling tools that include such materials.